Abstract
Retrosplenial cortex (RSC) lies at the interface between sensory and cognitive networks in the brain and mediates between these, although it is not yet known how. It has two distinct subregions, granular (gRSC) and dysgranular (dRSC). The present study investigated how these subregions differ with respect to their electrophysiology and thalamic connectivity, as a step towards understanding their functions. The gRSC is more closely connected to the hippocampal formation, in which theta-band local field potential oscillations are prominent. We, therefore, compared theta-rhythmic single-unit activity between the two RSC subregions and found, mostly in gRSC, a subpopulation of non-directional cells with spiking activity strongly entrained by theta oscillations, suggesting a stronger coupling of gRSC to the hippocampal system. We then used retrograde tracers to test for differential inputs to RSC from the anteroventral thalamus (AV). We found that gRSC and dRSC differ in their afferents from two AV subfields: dorsomedial (AVDM) and ventrolateral (AVVL). Specifically: (1) as a whole AV projects more strongly to gRSC; (2) AVVL targets both gRSC and dRSC, while AVDM provides a selective projection to gRSC, (3) the gRSC projection is layer-specific: AVDM targets specifically gRSC superficial layers. These same AV projections are topographically organized with ventral AV neurons innervating rostral RSC and dorsal AV neurons innervating caudal RSC. These combined results suggest the existence of two distinct but interacting RSC subcircuits: one connecting AVDM to gRSC that may comprise part of the cognitive hippocampal system, and the other connecting AVVL to both RSC regions that may link hippocampal and perceptual regions. We suggest that these subcircuits are distinct to allow for differential weighting during integration of converging sensory and cognitive computations: an integration that may take place in thalamus, RSC, or both.
Highlights
Retrosplenial cortex is a brain region lying between primary sensory cortices and the hippocampal spatial memory system
We only included rats in which we could identify the electrode track in the histological section, and for which histology indicated that electrodes sampled both Retrosplenial cortex (RSC) subregions
Histology As mentioned earlier, for the analysis we only included four rats for which histology indicated that electrodes sampled both RSC subregions
Summary
Retrosplenial cortex is a brain region lying between primary sensory cortices and the hippocampal spatial memory system. It has two component subregions, granular (gRSC) and dysgranular (dRSC), that may serve different, albeit related, functions (Aggleton et al, 2021). Retrosplenial cortex (RSC) is a fundamental component of an extended hippocampal circuit supporting learning and memory (Vann et al, 2009). While rodent studies have characterized the spatial functions of the RSC (Vann et al, 2009) the details are far from complete (Mitchell et al, 2018), directional processing is a prominent feature (Cho and Sharp, 2001; Lozano et al, 2017; Alexander et al, 2020; Chen et al, 1994)
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